This invention relates to an integrated CMOS auxiliary power supply switching circuit and more particularly to the utilization of such a circuit a circuit along with a read/write memory so as to form a non-volatile read/write memory module.
In order to maintain the data stored in read/write memory, the auxiliary battery must supply power to an associated read/write memory chip. However, a problem arises in preventing the read/write memory from drawing power from the battery while the read/write memory is being powered from a main system power supply. Additionally, a second problem, which arises when such a non-volatile memory system is coupled to the main system power supply in addition to other integrated circuits being coupled to the main system power supply and interface buses, is in isolation the auxiliary battery from the additional integrated circuits coupled to the main power supply without necessitating additional external components for this purpose. The problem of providing a non-volatile read/write memory is further complicated where the read/write memory module containing the auxiliary battery power supply is removeable from the main system, that is, is a plug in memory module which upon being plugged into the main system couples to the main power supply plus an interface bus. One solution to this problem is to isolate the auxiliary battery power supply from the main system power supply when the module is coupled to the main system by using a blocking diode. When the main system power supply is not present at the non-volatile memory module power supply bus (either when the main power supply is shut off or when the module is not coupled to the main power supply bus), the blocking diode will be forward biased and supply power to the read/write memory circuit so as to maintain valid data therein. A problem with this solution is that when the main power is switched off, the auxiliary battery power supply is coupled to the main system power supply bus and the whole system is powered from the auxiliary battery power supply, not just the read/write memory module. Typically, the blocking diode forms an additional component within the module housing the read/write memory integrated circuit and the auxiliary battery power supply.
A partial solution to the problem of powering additional system circuits when the main system power supply is not present and the non-volatile memory is coupled to the main system supply bus is to separate the power bus line into individual power lines, coupling to each module independently. However, the problem with this solution is that it requires extra printed circuit board space to run the extra power line or lines, thereby raising the cost of the system.
It is therefore an object of the present invention to overcome the basic problems involved with the previous systems for protecting each non-volatile memory module and the auxiliary battery contained therein from powering other integrated circuits in the system. In addition, it is an object of the present invention to reduce to the component count by eliminating the need for an external blocking diode and extra switches needed with previous solutions to the non-volatile read/write memory problems.